A steel material is cast, then worked hot and/or cold to be formed into the product shape, then is annealed. The annealed steel material is chemically treated or plated on its surface. In this case, if an oxide film is formed on the surface, the surface will not be sufficiently chemically treated or plated and the subsequent coatability, plating adhesion, and corrosion resistance will be impaired. Therefore, an annealed steel material has to be cooled in a nonoxidizing manner.
When cooling a steel material in a nonoxidizing manner, it is cooled by nitrogen or another nonoxidizing gas. The reason is that if the gas contains oxygen or another oxidizing gas, the steel material will be oxidized.
If using water as the cooling medium, since the water itself is oxidizing, it is not possible to avoid oxidation of the steel material. However, if the steel material is thick or a relatively fast cooling rate is necessary, the required cooling rate cannot be obtained by cooling using gas and therefore cooling using water becomes necessary. In this case, the oxide film formed on the steel material surface has to be removed after annealing by pickling or other post-treatment.
As the method of cooling a steel material by a nonoxidizing manner when water-cooling the material, the method of reducing the solute oxygen in the cooling water (deaerating it) has been proposed.
Japanese Patent Publication (A) No. 54-24211 proposes the method of using water once deaerated by boiling for the cooling, Japanese Patent Publication (A) No. 57-198218 proposes the method of reducing the solute oxygen concentration in the cooling water to 0.01 ppm or less, and further Japanese Patent Publication (A) No. 61-179820 proposes a cooling facility provided with a deaeration facility.
The oxidation of a steel material during water cooling includes oxidation proceeding using solute oxygen as its oxidizing source and oxidation by the cooling water itself, but in the above patent documents, it is proposed to simply reduce the solute oxygen without understanding their contributions.
Japanese Patent Publication (A) No. 63-7339 considers the fact that there is oxidation due to solute oxygen and water and proposes an electrochemical technique for reducing the oxidation by the water.
However, the prior art does not differentiate between the thickness of the oxide film due to the solute oxygen in the water and the thickness of the oxide film due to the steam generated due to contact with the heated steel material (that is, the cooling water itself), identify the factors affecting the thicknesses of the oxide films, and quantitatively clarify the relationship between the thicknesses of the oxide films and the affecting factors.